Letterbook Docs

What is DKIM? Why do we need it?

Learn how DKIM helps receiving mail servers trust messages sent by Letterbook on your behalf.

Last updated June 25, 2026

What DKIM means#

DKIM stands for DomainKeys Identified Mail. It is an email authentication standard that lets a sending service attach a cryptographic signature to an outbound message.

When Letterbook sends a reply from your support address, the receiving mail server can check that signature against a public DNS record on your domain. If the signature matches, the receiver has evidence that the message was authorized by the domain owner and was not modified after it was sent.

Why Letterbook needs DKIM#

Letterbook sends replies on behalf of your support address. Without DKIM, receiving mail servers have less evidence that those replies are legitimate. That can cause messages to look suspicious, land in spam, or fail authentication checks on stricter domains.

DKIM helps with three things:

Trust: receiving servers can verify that Letterbook is allowed to sign mail for your domain.
Deliverability: authenticated messages are less likely to be treated as spoofed mail.
Brand consistency: customers see replies from your support address instead of a generic sender.

How DKIM works with DNS#

DKIM uses a pair of keys:

A private key used by the sending service to sign outbound messages.
A public key published in your domain's DNS records.

Letterbook handles the private signing side. You publish the DNS records Letterbook provides so receiving mail servers can verify the signature.

DKIM, SPF, and DMARC#

DKIM is one part of email authentication.

DKIM verifies that a message was signed by an approved sender for the domain.
SPF lists which mail servers are allowed to send for the domain.
DMARC tells receivers what to do when authentication checks fail.

For reliable sending, all three should be aligned with the domain customers see in the sender address.

Why this matters for support#

Support replies are customer-visible and often include billing, account, or troubleshooting instructions. If those replies fail authentication, customers may never see them, or they may distrust the message when they do.

DKIM also helps protect your brand. It gives receiving mail systems a way to distinguish authorized Letterbook replies from messages that only pretend to come from your domain.

Auto configure versus manual setup#

If your DNS is hosted in Cloudflare, Letterbook can use Domain Connect to request the required records automatically. You approve the DNS changes in Cloudflare, then Letterbook verifies the domain.

If your DNS is hosted somewhere else, copy the records from Letterbook and add them manually in your DNS provider.